It is well known in the art of injection molding to provide a melt distribution apparatus in association with an injection mold for sake of distribution a molding material that is fluidly receivable in the melt distribution apparatus, in use, from an injection unit, for distribution to one or more molding cavities that are defined in the injection mold. There are varied types of melt distribution apparatus in use today. One such type of the melt distribution apparatus is commonly known to those skilled in the art as a hot runner.
The hot runner is characterized in that a network of melt channels defined therein, for the distribution of the molding material, are typically heated such that the molding material that is flowable therethrough, in use, is prevented, for the most part, from solidifying therein. The hot runner typically includes a housing made up of plates and wherein the plates are adapted to define one or more cavities therebetween and/or channels therethrough within and/or through which are arranged various heated melt distribution components such as, for example, a manifold, a nozzle drop and/or a sprue bushing that together cooperate to define the network of melt channels. The nozzle drop of the hot runner defines, in use, at least one melt channel of the network of melt channels and further is fluidly connectable with the molding cavity of the injection mold. As the flow of the molding material through the nozzle drop is typically cyclically started and stopped with each molding cycle, a gating structure is also typically associated with the nozzle drop, the gating structure being configured to selectively block a gate to control an outflow of the molding material from the nozzle drop. The gating structures are typically thermally and/or mechanically operable. One such mechanically-operable gating structure that is used extensively in the industry is the so-called valve-gate apparatus. The valve-gate apparatus uses a valve member that is selectively actuatable, in use, by a valve gate actuator, for selectively blocking of the gate. Also typically associated with the nozzle drop having a valve-gate apparatus is a back-up device that is arranged between the housing and the manifold. The back-up device is configured to couple, in use, an operationally generated force between the manifold and the housing. The back-up device may also be configured to thermally separate, in use, the heated manifold from the relatively cool housing thereby making it easier to maintain the manifold and/or other heated melt distribution component at a desired operating temperature. Further, the back-up device may be configured to define a guiding structure for slidably guiding, in use, a portion of the valve member as the valve member is selectively slidably cycled between a blocked and an open configuration with respect to the gate.
A hot runner including a valve-gate apparatus that includes a back-up device is described in U.S. Pat. No. 6,840,758 to Babin et al., published on Jan. 11, 2005. The patent describes, amongst other things a hot runner that includes a spacer that may be used to thermally insulate a manifold block from an actuator block. The spacer may also be used to help retain a bushing in place in a manifold pass-through and to improve the seal between the bushing sealing surface on bushing and a manifold sealing surface.
With the valve-gate apparatus, in general, it is not uncommon for molding material, such as a thermoplastic (e.g. Polyethylene Terephthalate (PET) and the like), in either a solid particulate or in a molten state, to propagate rearward through whatever clearance that may be present between the valve member and the guiding structure and/or between the valve member and an associated valve bushing. The valve bushing is configured to be arranged in the manifold for slidably guiding, in use, a further portion of the valve member. This undesirable propagation of molding material may eventually impede the slidable movement of the valve member or continue further into the valve-gate apparatus wherein it may effect the operation of other components thereof (e.g. a pneumatic valve gate actuator).